1. Field of the Invention
This invention relates to hydraulic control systems, and more particularly to hydraulic systems in which a plurality of hydraulic actuators are to be precisely positioned in dependence on the magnitude of a similar plurality of electrical control signals.
2. Description of the Prior Art
There are numerous such control systems, and this invention would represent a significant advantage in connection with many of them. One exemplary and very significant application of such controls is in aircraft systems where hydraulic controls are provided for adjusting mechanical variables in jet aircraft engines. The gas turbine engines which are used to power conventional jet aircraft have commonly used hydraulic actuators for control of air valves, fuel valves, engine variable geometry, and the like. As engine designers attempt to achieve more and more performance from the gas turbine, the number of hydraulic actuators has increased significantly, and may approach 17 in number. Even gas turbine engines used on older commercial aircraft typically have on the order of six hydraulic actuators.
Heretofore, each hydraulic actuator was provided with a device to convert an electrical input signal into a mechanical position of the actuator. Most typically, that had been done with a torque motor connected to and driving a hydraulic servo valve; the servo valve, in turn, controlled the supply of hydraulic fluid to the actuator. Both torque motors and servo valves are fairly expensive, and both are fairly weighty components, particularly for aircraft applications where weight savings on the order of pounds can translate into substantial operating cost savings over the life of the aircraft.
Applicant is aware of a concept having been proposed to reduce weight and cost in such systems, by using a single pilot valve multiplexed among a plurality of actuators. In substance, the pilot valve has a spool which is rotated for multiplexing, and which is positioned vertically by the torque motor to establish a control position. The spool and valve would be modified to provide a plurality of ports at different angular positions of the spool such that the vertical control position of the valve combined with a plurality of angular multiplex positions could be used to sequentially deliver hydraulic fluid to a plurality of actuators.
It is applicant's belief that a system of that type could not be reduced to practice for any but the most rudimentary systems because of a number of limitations, the most prominent one being the substantially reduced flow rate to any given actuator for a servo valve of any reasonable size. The flow rate reduction is a result of two factors--(1) reduced flow through a pilot valve which is configured as a multiplexer, and (2) the fact of multiplexing itself which has flow going to an actuator only during its time slot. Thus, while in principle the system might work in applications where speed of response and fineness of control are not important criteria, in a jet engine control, for example, the concept would not appear to be workable.
Multiplexing of hydraulic circuits is not broadly new. It can be used for example in sharing a single transducer among a number of hydraulic or pneumatic channels, such as illustrated in Moore et al. U.S. Pat. No. 3,645,141. The opportunity to share a control servo valve among multiple actuators is also suggested in the literature, but not on a simultaneous real time basis, insofar as applicant is aware. Applicant, however, knows of no application where multiplexing has been successfully used in control of high performance hydraulic systems such as for gas turbine engine control, where the requirements are for precise position control, a wide range of controllable actuator speeds, and a demand, at least for some channels, of high speed controlled movement of the actuator. Thus, while a pressure sensing application (e.g., Moore) can be configured to share a single transducer among multiple channels, because no substantial fluid flow is required for that application, and while in low performance applications it may be possible to selectively connect different hydraulic circuits to a single servo valve, it has not heretofore been possible to accurately control a plurality of high performance actuators which require substantial fluid flow to generate adequate force or sufficient rate of movement in a hydraulically multiplexed system. It is for those reasons, perhaps among others, that designers have traditionally thought in terms of one control for one actuator in applications like aircraft engines where a plurality of such actuators must be capable of simultaneous action and have a relatively high fluid flow rate needed in order to meet performance requirements.
The concept of hydraulic amplification is also known, and is used, for example, where the flow rate required by an actuator is much higher than the flow capacity of an associated pilot valve. One example of a hydraulic amplification device is the dog valve in which a relatively low flow rate pilot apparatus controls the position of a follower which in turn controls a much higher flow rate. The pilot and follower, by virtue of their mechanical association, have built-in mechanical feedback which helps to assure system stability. While the dog valve can thus achieve hydraulic amplification, it has been applied only, insofar as applicant is aware, in a one valve per channel implementation, thus requiring a complete dog valve and actuator assembly per channel to be controlled.